Climate change is scientifically incontrovertible and has become a defining problem for the current as well as future generations. The Paris agreement to mitigate climate change  was a truly historic agreement that signaled to the entire world that mitigation of climate change is an urgent priority among leaders of the nations of the world. What the world urgently needs now are scalable solutions for bending the curve — flattening the upward trajectory of human-caused greenhouse gas emissions and consequent global climate change (Figure 2). The overall targets for stabilizing climate change are rather straightforward and have been prescribed in numerous studies . Basically energy consumption has to become carbon neutral as soon as possible and in addition we have to drastically mitigate emissions of numerous other climate warming pollutants within few decades [3, 4]. However, the specific pathways or solutions to reach these targets are complex and require behavioral, institutional, technological and governance changes, and these have not been prioritized nor synthesized into one logical framework. Furthermore the solutions have to be based upon real world examples of the art of the possible and prioritize solutions that are scalable to the whole world. The multi-dimensional nature of the problem requires inter-disciplinary as well as cross-disciplinary collaboration for crafting a set of solutions to Bend the Curve of carbon emissions and climate change.
Towards this ambitious goal, fifty researchers and scholars (UC-Fifty) — from a wide range of disciplines across the University of California system — formed a climate solutions group and came together in 2015 to identify these solutions, many of which emerge from UC research as well as the research of colleagues around the world. Taken together, these ten solutions can bend the curve of climate change. The 10 scalable solutions, described here, present pragmatic paths for achieving carbon neutrality and climate stability in California, the United States and the world. The 10 solutions were derived from detailed analyses of the climate change problem as well as its multi-dimensionality by the UC-Fifty. These analyses and resulting recommendations are described in 8 companion papers in this special volume. The companion papers fall under five categories: I. Science Solutions Cluster; II. Societal Transformation Solutions Cluster; III. Governance Solutions Cluster; IV: Market- and Regulations-Based Solutions Cluster; and V. Technology-Based Solutions Cluster.
The effort by the UC-Fifty is inspired by California’s recent pledge to reduce carbon emissions by 40 percent below 1990 levels by 2030 , and by the University of California’s pledge to become carbon neutral by 2025 . What is taking place in California today is exactly the sort of large-scale demonstration project the planet needs. And this statewide demonstration project is composed of many of the kinds of solutions that can be scaled up around the world.
California has provided a remarkable example for the world by achieving dramatic reductions in air pollution, while continuing to grow economically . Furthermore, the air pollution control industry in California generated $6.2 billion in revenues and employed 32,000 people in 2001 . In this study, we propose a set of strategies for combating climate change and growing the economy in California, the nation and the world, while building present-day and intergenerational wealth, and improving the well-being of people and the planet. The University of California has played a key role in California’s pioneering leadership in energy and environmental policy through research, teaching and public service, and currently is partnering with local, state, federal and international leaders in the public, private and philanthropic sectors to address our pressing climate change challenges (e.g, ). We still have much more to do here in California. We are eager to share these lessons with the world and together build a better, safer, healthier and more equitable world, while bending the curve of climate change. As we make the changes necessary to achieve carbon neutrality at the University of California, employing solutions that can be scaled up to developing energy and climate solutions for the world, hundreds of thousands of faculty, students and staff across our 10 campuses and three affiliated national laboratories will be learning and sharing with the world how we can bend the curve of greenhouse gas emissions and stop global warming through taking bold yet pragmatic steps and lowering the barriers so others can follow.
This is evident in the increased frequency and intensity of storms, hurricanes, floods, heat waves, droughts and forest fires [10, 11]. These extreme events, as well as the spread of certain infectious diseases, worsened air pollution, drinking water contamination and food shortages, are creating the beginning of what soon will be a global public health crisis. A whole new navigable ocean is opening in the Arctic. Sea levels are rising, causing major damage in the world’s most populous cities. All this has resulted from warming the planet by only about 0.9 ºC, primarily from human activities . Since 1750, we have emitted 2 trillion metric tons of carbon dioxide (CO2) and other greenhouse gases. The emission in 2011 was around 50 billion tons and is growing at a rate of 2.2 percent per year . If this rate of increase continues unabated, the world is on target to warm by about 2 ºC in less than 40 years [3, 4]. By the end of the century, warming could range from 2.5 ºC to a catastrophic 7.8 ºC . We are transitioning from climate change to climate disruption. With such alarming possibilities the planet is highly likely to cross several tipping points within decades, triggering changes that could last thousands of years . All of this is occurring against a backdrop of growing needs and pressures by humans, as our population is set to increase by at least 2 billion people by 2050.
Bending the curve refers to flattening the upward trajectory of human-caused warming trends. Reducing CO2 emissions by 80 percent by 2050 and moving to carbon neutrality post-2050 would begin to bend the temperature curve downward and reduce overall warming by as much as 1.5 ºC by 2100 [11, 13]. Temperature estimates for future warming trends as well as for the mitigated warming given throughout this study have a 95 percent probability range of ±50 percent. For example, a value of 2 ºC given here is the central value with a 95 percent range of 1 to 4 ºC. That is, there is a 95 percent probability the true value will be within that range.
More rapid reductions can be achieved by reducing four short-lived climate pollutants. These short-lived climate pollutants, known as SLCPs, are methane (CH4), black carbon, hydrofluorocarbons (HFCs, which are used in refrigerants) and tropospheric ozone. If currently available technologies for reducing SLCPs were fully implemented by 2030, projected warming could be reduced by as much as 0.6 ºC [3, 13, 14] within two to four decades, keeping the mid-century warming well below 2 ºC relative to the pre-industrial average. This could give the world additional time to achieve net-zero emissions or even negative carbon emissions through scaling up existing and emerging carbon- neutral and carbon sequestration technologies and methods. Achieving both maximum possible mitigation of SLCPs and carbon neutrality beyond 2050 could hold global warming to about 2 ºC through 2100, which would avert most disastrous climate disruptions. This is our goal in this study.
In what follows, we describe 10 practical solutions to mitigate climate change that are scalable to the state, the nation and the world. There are many such reports offering recommendations and solutions to keep climate change under manageable levels. We take full account of such action-oriented reports and offer some unique solutions to complement them. Many of the solutions proposed here are being field tested on University of California campuses and elsewhere in California. The background, the criteria, the quantitative narrative and justification for these solutions can be found in the companion papers in this special volume.
In the economic boom following World War II — fueled by large increases in population, vehicles, diesel trucks and coal-burning industries — California recorded some of the highest air pollution levels, competing with the city of London for the dubious title of the worst polluted region in the world. Since then, California has made a remarkable turnaround. From 1960 to the present, California has reduced levels of particles and gases related to air pollution by as much as 90 percent .
The concentration of black carbon was reduced by 90 percent across California. In the meantime, fuel consumption for the transportation sector increased by a factor of five and population grew from 15.5 million (1959) to 39 million (2014). California also has made impressive gains in energy efficiency and in lowering its carbon footprint. Its per capita energy consumption is among the lowest in the United States (48th) and its per capita electricity consumption is the lowest — roughly half of the U.S. per capita consumption [16, 17].
California is one of the most energy- efficient and greenest economies in the world. It is the second-to-least carbon-intense economy in the world next to France, which relies heavily on nuclear power. It also is a leader in renewable power generation with 23 percent of its electricity generated from renewables (not including hydropower), second only to Germany (which generates 27 percent of its electricity from renewables). These impressive environmental gains did not hurt California’s economy, which grew at an impressive pace with the highest gross domestic product of all states in the nation, constituting the world’s eighth largest economy. California has shown how to reduce fossil fuel related pollution emissions while sustaining strong economic growth.
Emboldened by this favorable experience in regulating air pollution, California in 2002 passed the first law in the country that targeted greenhouse gas emissions from vehicles. In 2006, it enacted the precedent-setting Global Warming Solutions act and gave authority to California’s air pollution agency, the California Air Resources Board (CARB), to enact policies to reduce its greenhouse gas emissions to 1990 levels by 2020. The state responded with a suite of measures that include a cap and trade program, a low carbon fuel standard for vehicles, automobile emission standards expected to reduce emissions by 30 percent by 2016, renewable portfolio standards for utilities, energy efficiency programs for buildings and appliances, and transit and land use programs to reduce vehicle miles traveled. This has been followed by another milestone in 2015 when Gov. Brown issued an executive order setting a goal of reducing CO2 emissions to 40 percent below 1990 levels by 2030, which is the pathway required for stabilizing climate below 2 ºC relative to the pre-industrial average. The legacy of California’s air quality and energy efficiency programs since the 1960s and the depth of expertise at CARB on the multi-dimensional aspects of climate change mitigation have placed California in a unique position to embark on such ambitious low carbon pathways.
While its geography, equable climate and commerce have favored green growth, this progress came as a result of five decades of consistent and innovative policies that relied on sound research, innovative development and aggressive implementation of policies. While California relied only on command and control regulation until the 1990s, the state began rolling out market incentives for controlling nitrous oxide emissions and demonstrated the efficacy of market instruments to mitigate certain types of emissions. Relying on this experience, CARB launched a cap and trade system in 2013 to reduce carbon emissions from utilities, industrial facilities and fuel distributors, covering 85 percent of California’s emissions, making it the most comprehensive cap and trade market in the world .
California cannot address climate change on its own, but the state can serve as a living laboratory for “the art of the possible,” sharing its good practices and cooperating with other states and nations to mitigate their emissions . To achieve this goal, California has created an “Under 2 MOU,”  an agreement Gov. Brown co-founded with the state of Baden-Württemberg in Germany. The “Under 2 MOU” is an agreement among subnational jurisdictions around the world to limit the increase in global average temperature to below 2 ºC. Since the global agreement was first signed in May 2015, a total of 45 jurisdictions in 20 countries and five continents, with a total GDP of US $14 trillion, have signed or endorsed the agreement.
This study is an outgrowth of the University of California President’s Carbon Neutrality Initiative. The authors of this study and our colleagues at the University of California’s 10 campuses and three affiliated national laboratories are strongly motivated by the special demands of this ambitious goal, and we are also motivated by corresponding goals for the state of California, the nation and the world. The UC Carbon Neutrality Initiative is dedicated to achieving net-zero greenhouse gas emissions by 2025 across all 10 UC campuses. It should be emphasized that a net- zero emission target is enormously demanding and requires careful strategic planning to arrive at a mix of technologies, behavioral measures and policies, as well as highly effective communication — all of which, taken together, are far more challenging than simply reducing emissions by some 40 percent or even 80 percent. Each campus has a unique set of requirements based on its current energy consumption and emissions. Factors such as a local climate, reliance on cogeneration facilities, access to wholesale electricity markets and whether the campus has a hospital and medical school, shape the specific challenges of the campuses, each of which is a “living laboratory” for learning and adapting.
Examples of current projects related to the Carbon Neutrality Initiative are described in the companion papers. These include an 80 megawatt solar array in the Central Valley (the largest at any U.S. university), an experimental anaerobic digester that is using food waste to produce bio-methane, a large fuel cell that generates 2.8 megawatts of electricity from a municipal waste water treatment facility, smart lighting and smart building systems that dramatically reduce energy consumption and a solar greenhouse that selectively harvests light for solar electricity. These and other works at the University of California illustrate the commitment that we have made to mitigate climate change.
These 10 pragmatic, scalable solutions — all of which can be implemented immediately and expanded rapidly — will clean our air and keep global warming under 2 ºC and, at the same time, provide breathing room for the world to fully transition to carbon neutrality in the coming decades. More details on each solution can be found in the companion chapters to follow in this special volume.
Of the 10 solutions proposed here, seven (solutions #1 and #4 through #9) have been or are currently being implemented in California (see section 1.4).
California’s experience provides valuable lessons, and in some cases direct models, for scaling these solutions to other states and nations. Decades of research on University of California campuses and in national laboratories managed by the university contributed significantly to the development of these solutions. Several of the renewable energy technology solutions in solutions #6 and #7 have been field tested on University of California campuses (see section 1.5). Scaling these solutions to other states and nations and eventually globally will require attitudinal and behavioral changes covered in solutions #2 and #3.
UC researchers currently are working on many of these solutions, along with colleagues around the world. UC faculty also are involved in research on solution #10 to identify and improve carbon sinks in natural and managed ecosystems by expanding existing, proven practices worldwide. The cost of fully implementing these solutions will be significant, but California shows that it can be done while maintaining a thriving economy. And the cost is well justified in light of the social costs of carbon emissions, including 7 million deaths every year due to air pollution linked to fossil fuel and biomass burning which also releases climate warming pollutants to the atmosphere.
If we can scale these 10 solutions beginning now, we can dramatically bend the curve of deadly air pollution and global warming worldwide (Table 1). California can’t bend the curve on its own. Neither can the University of California. But we can be part of powerful networks and collaborations to scale these solutions.
|Solutions by Topical Cluster||CA’s Climate Strategy & Estimated Benefits||Potential Climate Strategy & Benefits for the World|
|Solution 1: SLCPs and carbon neutrality: Reduce short-lived climate pollutants (SLCPs) and replace current fossil-fueled energy systems with carbon neutral technologies||CA’s key targets to reduce greenhouse gas (GHG) emissions:
The State is currently on track to achieve its reduction of 40% GHG by 2030 under state Assembly Bill 32; however, more will need to be done to achieve 80% reductions by 2050.
[[Globally these efforts would save as many as 100 million lives lost to air pollution by 2050]]
|Societal Transformation; Governance; and Market- and Regulation-Based Solutions|
|Societal Transformation Solutions||
Solutions 2–6 are essential to obtain public support for the decisive actions required for carbon neutrality. These can variably work in tandem with solutions #1, 7, 8, 9, and 10 to achieve emissions reductions.
||California leads the way in providing Solutions for other Subnational and National Jurisdictions and their Governments:
|Solution 2: Attitudinal and behavior change: Foster a global culture of climate action through coordinated public communication and education.|
|Solution 3: Climate collaboration: design venues where stakeholders converge around concrete problems|
|Solution 4: Subnational models of governance and collaboration:|
|Solution 5: Adopt market-based instruments to create efficient incentives businesses and individuals for to reduce CO2 emissions.|
|Market- and Regulation- Based Solutions|
|Solution 6: Narrowly target direct regulatory measures at high emissions sectors not covered by market-based policies|
|Solution 7: Promote immediate widespread use of mature technologies such as photovoltaics, wind turbines, battery and hydrogen fuel cell electric light duty vehicles, and more efficient end-use devices, especially in lighting, air conditioning, appliances and industrial processes||Demonstration of technology in California has made policies and implementation feasible: Zero emission vehicles program: first developed in the 1990s, successful demonstrations today are making it possible to ramp up zero emission vehicle policies not possible earlier. As a technologies improve for renewables, Renewable Portfolio Standards (RPS) ramp-up becomes feasible. First piloted in the 1990s, successful demonstrations are making scalability possible. UC demonstrations include an 80 megawatt solar array, an experimental anaerobic digester that is using food waste to produce bio-methane, a large fuel cell that generates 2.8 megawatts of electricity from a municipal waste water treatment facility, smart lighting and smart building systems that dramatically reduce energy consumption and a solar greenhouse that selectively harvests light for solar electricity.
The program will combine climate investments within a local area for catalytic impact, including investments in energy, transportation, active transportation, housing, urban greening, land use, water use efficiency, waste reduction, and other areas, while also increasing job training, economic, health and environmental benefits.
|Together solutions #7 and 8 are necessary for achieving worldwide carbon neutrality post-2050.|
|Solution 8: Aggressively support and promote innovations essential for meeting the target of 80 percent reduction in CO2 emissions by 2050.(energy and transit electrification; building efficiency, energy storage, etc.)|
|Solution 9: Methane and black carbon reduction & HFCs phase-out||Pursuant to Chapter 523, Statutes of 2014 (SB 605), the Air Resources Board has developed a plan that calls for a 50% reduction in black carbon and fluorinated gas emissions and a 40% reduction in methane emissions by 2030.
Reducing methane emissions from landfills will be a key component of the short lived climate pollutant strategy. A key to achieving these goals is the successful collection and recycling of organic and other materials.
|A global reduction of methane emissions 50% and black carbon emissions 90%, would provide immediate reductions in global greenhouse effects and avoid crossing over tipping points within next three decades|
|Natural and Managed Ecosystem Solutions|
|Solution 10: Control deforestation, support forest recovery and agroforestry production systems, reduce food waste and energy recovery||Reducing methane emissions from landfills will be a key component of the short lived climate pollutant strategy. A key to achieving these goals is the successful collection andrecycling of organic and other materials.
The Governor’s 2016–17 budget notes that, in addition to increasing the frequency and severity of the state’s wildfire risk, an estimated 22M drought-striken, dead and dying trees compromise the carbon sequestration capabilities of the state’s forested lands.
|Forests can offset 20% of U.S. fossil fuel emissions (15); Controlling Amazon de-forestation by 70% avoids emitting 3.2 GTs CO2 (16); tropical forest regrowth absorbs 1.64 GTs of carbon per year (17); regrowth rates ~12–20 times that of old growth (18)|
This collaborative study is one of the first such effort that treats mitigation of air pollution and climate disruption under one framework. The solutions proposed here recognize the fact that fossil fuel combustion — which produces greenhouse gases — also produces particles and gases such as ozone and black carbon, which also contribute to global warming. Others, such as sulfates, cause sunlight to dim and dry the planet. We can accelerate solutions and gain some time for long-term change to a carbon-neutral world by bending the curve of all of these pollutants immediately and simultaneously as part of one unified strategy.
These 10 solutions leverage the power of concern for human health worldwide. People care about human health. Burning fossil fuels causes both air pollution and climate changes that result in human illnesses and death. As the Lancet Commission concluded in June 2015: “The effects of climate change are being felt today and future projections represent an unacceptably high and potentially catastrophic risk to human health” .
This study recognizes that intra- regional, intra-generational and inter-generational equity and ethical issues are inherent in climate change and any solutions to climate change. These issues arise in part because consumption by about 15 percent of the world’s population contributes about 60 percent of climate pollution; while 40 percent of the population, who contribute very little to this pollution, as well as generations unborn, are likely to suffer the worst consequences of climate disruption. These solutions represent an integrated approach that includes familiar goals for achieving carbon neutrality through renewable energy, with new goals for reducing SLCPs immediately; building on California’s success to encourage sub-national governance, regulations and market-based instruments; and innovative approaches in education, communication and incentives to encourage attitudinal and behavioral changes. To be effective, this integrated strategy requires engagement by diverse stakeholders and the creation of a culture of climate action through localized interventions that lower barriers for citizens to take concrete steps to participate in solving our climate crisis.
These solutions recognize the fact that fundamental changes in human attitudes and behaviors toward nature and each other are critical for bending the curve of air pollution and global warming. As a result, two of the solutions deal with bringing researchers and scholars together with community and religious leaders and stakeholders to lower barriers to addressing climate change from the local level on up.
The study also recognizes the fundamental importance of effective communication to reach and engage diverse constituencies throughout the world to bend the curve of emissions and warming, achieve carbon neutrality and stabilize Earth’s climate.
Our 10 scalable solutions are grouped in six clusters listed below.
The intra-regional, intra-generational and inter-generational equity issues of climate change raise major questions of ethics and justice. These questions compel us to reflect deeply on our responsibility to each other, to nature, and to future inhabitants of this planet — Homo sapiens and all other living beings alike. It is for these reasons that societal transformation merits such high ranking in this study, even above regulatory and technological solutions. Top-down action will be difficult to implement without substantial support from the general public, which can be accelerated by societal transformations from the bottom up.
The problem of emissions won’t solve itself. Policy makers must send decisive signals to firms and individuals. So far, very few places in the world have adopted strong greenhouse gas mitigation policies. California is an exception, but California is less than 1 percent of the global problem. If we are to lead, we need to adopt policies that others can emulate; this is tricky because the best policies will vary with local circumstances. In general, there are two flavors of emissions policies: direct regulation and market- based (cap and trade and carbon pricing) regulation.
Economic theory and empirical evidence tell us that market approaches are more cost-effective. In a few cases where market based control systems have been used at scale — such as trading of lead pollution, trading of sulfur dioxide pollution, and European and Californian carbon markets — that theory is borne out by evidence. Yet it is already clear that market approaches are politically very difficult to implement in part for the very reasons that many analysts find them attractive: They make the real costs of action highly transparent .
As a matter of policy design, we have chosen not to come down in favor of either market based or regulatory approaches, but to include both. Specifically, we recommend the following:
The technological measures under solutions #7 and #8, if fully implemented by 2050, will reduce global warming by as much as 1.5 ºC by 2100, and combined with measures to reduce SLCPs in solution #9 will keep warming below 2 ºC during the 21st century and beyond.
Global emissions of CO2 and other greenhouse gases in 2010 totaled 49 gigatons of equivalent CO2 per year, with 75 percent due to increases in CO2 and 25 percent from other greenhouse gases. This estimate from the IPCC 2013  does not include two of the SLCPs, ozone and black carbon. About 32 gigatons per year are due to CO2 from fossil fuels and industrial processes. The challenge for technology solutions is to bring down emissions of CO2 to less than 6 gigatons per year by 2050, and reduce the emissions of methane and black carbon by 50 percent and 90 percent respectively by 2030. This in turn will reduce ozone levels by at least 30 percent. In addition, HFCs must be phased out completely by 2030. To indicate the importance of these non- CO2 mitigation measures: HFCs are the fastest growing greenhouse gases; if emissions continue to grow at current rates, HFCs alone will warm the climate by 0.1 ºC by 2050 and 0.5–1.0 ºC by 2100.
|Extended pre-mine degasification and recovery and oxidation of CH4 from ventilation air coal mines
Extended recovery and utilization, rather than venting, of associated gas and improved control of unintended fugitive emissions from production of oil and natural gas
Reduced gas leakage from long-distance transmission pipelines
|Extraction and transport of fossil fuels|
|Separation and treatment of biodegradable municipal waste through recycling, composting and anaerobic digestion as well as landfill gas collection with combustion/utilization
Upgrading primary wastewater treatment to secondary/tertiary treatment with gas recovery and overflow control
|Control of CH4 emissions from livestock, mainly through farm-scale anaerobic digestion of manure from cattle and pigs
Intermittent aeration of continuously flooded rice paddies
|BC measures (affecting BC and other co-emitted compounds)|
|Diesel particle filters for road and off-road vehicles
Elimination of high-emitting vehicles in road and off-road transport
|Replacing coal by coal briquettes in cooking and heating stoves
Pellet stoves and boilers, using fuel made from recycled wood waste or sawdust, to replace current wood-burning technologies in the residential sector in industrialized countries
Introduction of clean-burning biomass stoves for cooking and heating in developing countries1, 2
Substitution of clean-burning cookstoves using modern fuels for traditional biomass cookstoves in developing countries1, 2
|Replacing traditional brick kilns with vertical shaft kilns and hoffman kilns
Replacing traditional coke ovens with modern recovery ovens, including the improvement of end-of-pipe abatement measures in developing countrie
|Ban on open field burning of agricultural waste1||Agriculture|
The invention of the steam engine and the subsequent acquisition of breathtaking technological prowess culminating in the current information age two centuries later have led to enormous improvements in human well- being. But the impressive improvement has come at a huge cost to the natural environment. The combination of air and water pollution, species extinction, deforestation and climate change has become an existential threat to life on this planet. The gargantuan transformation of the environment has stimulated ecologists and geologists to consider whether the Holocene epoch — the past 12,000 years of relatively constant climate and environmental conditions that stimulated the development of human civilization — has ended, and a new epoch, the Anthropocene, has begun, an epoch that recognizes that human exploitation of Earth has become akin to a geologic force .
Most of the changes listed in Table 3, and many others, have occurred in a span of time equivalent to a human lifetime beginning in the 1950s, which is considered the beginning of the so-called “great acceleration” of human impacts. This also is the period that has seen the steepest increase in global mean temperatures, global pollution and deforestation.
|Human activity||Increase in size|
|World population||Increased six-fold|
|Urban population||Increased thirteen-fold|
|World economy||Increased fourteen-fold|
|Industrial output||Increased forty-fold|
|Energy use||Increased sixteen-fold|
|Coal production||Increased seven-fold|
|Carbon dioxide emission||Increased seventeen-fold|
|Sulfur dioxide emission||Increased thirteen-fold|
|Lead emission||Increased eight-fold|
|Water use||Increased nine-fold|
|Fish catch||Increased thirty-five fold|
|Blue whale population||99 percent decrease|
The greenhouse gas CO2 contributes about 50 percent to the manmade heat added to the planet. The other 50 percent is due to several other greenhouse gases and particles in soot. Those greenhouse gases include nitrous oxide, methane, halocarbons (CFCs, HCFCs and HFCs), and tropospheric ozone. The warming particles in soot are black carbon and brown carbon . The sources of these pollutants include fossil fuels (ozone, methane, black carbon), agriculture (methane and nitrous oxide), organic wastes (methane), biomass cooking and open burning (black and brown carbon) and refrigeration (halocarbons). Among these pollutants, the SLCPs (methane, black carbon, tropospheric ozone and HFCs) have lifetimes of days (black carbon) to 15 years (HFCs), which are much shorter than the century or longer lifetimes of CO2 and nitrous oxide.
When we add up the warming effects of CO2 with the other greenhouse gases, the planet should have warmed by about 2.3 ºC, instead of the 0.9 ºC observed warming. About 0.6 ºC of the expected warming is still stored in the deep oceans (to about 1,500 meters). That heat is expected to be released and contribute to atmospheric warming in two to four decades. The balance of 0.8 ºC involves a complication due to air pollution particles. In addition to black and brown particles (which warm the climate), fossil fuel combustion emits sulfate and nitrate particles, which reflect sunlight like mirrors and cool the planet. The mechanisms of warming and cooling are extremely complex. But when we add up all of the effects, sulfate and nitrate particles have a net cooling effect of about 0.8 ºC (0.3–1.2 ºC range). Summing 0.9 ºC of observed warming, 0.6 ºC stored in the oceans, and the 0.8 ºC masked by particles, adds up to the 2.3 ºC warming we should have seen from the build up of greenhouse gases to-date.
The particle cooling effect of 0.6 ºC should not be thought of as offsetting greenhouse gas warming. This is because the lifetimes of these particles last just days, and when stricter air pollution controls worldwide eliminate the emission of these particles, the 0.6 ºC cooling effect will disappear. This however does not imply that we should keep on polluting, since air pollution leads to 7 million deaths worldwide each year, as well as reductions in precipitation and decreases in crop yields.
Of the CO2 released to the air, 44 percent remains for a century or longer; 25 percent remains for at least a millennium. Due to fast atmospheric transport, CO2 envelopes the planet like a blanket. That blanket is growing thicker and warmer at an accelerating pace. It took us 220 years — from 1750 to 1970 — to emit about 1 trillion tons of CO2. We emitted the next trillion in less than 40 years. Of the total 2 trillion tons humans have put into the atmosphere, about 44 percent is still there. At the current rate of emission — 38 billion tons per year and growing at a rate of about 2 percent per year — the third trillion will be added in less than 20 years and the fourth trillion by 2050.
How does the CO2 blanket warm the planet? It works just as a cloth blanket on a cold winter night keeps us warm. The blanket warms us by trapping our body heat. Likewise, the CO2 blanket traps the heat given off by the Earth’s surface and the atmosphere. The surface and atmosphere absorb sunlight and release this solar energy in the form of infrared energy, some of which escapes to space. The human-made CO2 blanket is very efficient at blocking some of this infrared energy, and thus warms the atmosphere and the surface.
How large? Each trillion tons of emitted CO2 can warm the planet by as much as 0.75 ºC. The 2 trillion tons emitted as of 2010 has committed the planet to warming by 1.5 ºC. The third trillion we would add under business-as-usual scenarios would commit us to warming by 2.25 ºC by 2030.
How soon? A number of factors enter the equation. To simplify, we likely will witness about 1.5 ºC (or two-thirds of the committed warming) by 2050, mostly due to emissions already released into the atmosphere (although that amount of warming could come as early as 2040 or as late as 2070). By 2050, under a business-as-usual scenario, we will have added another trillion tons and the 2050 warming could be as high as 2 ºC — and the committed warming would be 3 ºC by 2050.
What is our predicament? We get deeper and deeper into the hole as time passes if we keep emitting at present rates under business-as-usual scenarios. The problem is that CO2 stays in the atmosphere so long; the more that is there, the hotter Earth gets. If we wait until 2050 to stop emitting CO2, there would be no way to avoid warming of at least 3 ºC because the thickness of the blanket covering Earth would have increased from 900 billion tons (as of 2010) to about 2 trillion tons (in 2050). Our predicament is analogous to stopping a fast-moving train: You have to put on the brakes well in advance of the point you need to stop; otherwise you will overshoot the mark.
A projection such as 2 ºC warming by 2050 is subject to a three-fold uncertainty range. It is important to note, however, that the uncertainty goes both ways: Things could be a little better than the average expectation, or a lot worse. The most disturbing part of the uncertainty is that it has a so-called “fat tail,” that is, a probability of a warming two to three times as much, or even more, than the 2 ºC that would result from best- case greenhouse gas mitigations. For example, the IPCC (2013 report) gives a 95 percent confidence range of 2.5–7.8 ºC warming for the baseline case without any mitigation actions . A warming in the range of 4 to 7.8 ºC can cause collapse of critical natural systems such as the Arctic sea ice, the Asian monsoon system and the Amazon rain forest. Economists argue that our decisions should be guided by such extreme possibilities and that we should take actions to prevent them, much as we already do in requiring buildings to withstand earthquakes and automobile manufacturers to equip our cars with seat belts and air bags in the unlikely event of an accident.
Observations with satellites, aircraft, ships and weather balloons gathered over the past three decades are providing disturbing evidence of nonlinear amplification of global warming through feedbacks. This has raised concerns that continued warming beyond 2 ºC can lead to crossing over tipping points in the climate system itself or in other natural and social systems that climate influences. Examples of climate-mediated tipping points include depletion of snowpack, drought, fires and insect infestations threatening whole forests, and the opening of new oceans in the Arctic. The following are among the many major feedbacks for which we have empirical evidence.
Observations from 1979 to 2012 reveal that warming in the Arctic has been amplified by 100 percent due to a feedback (a vicious cycle) between surface warming, melting sea ice and increased absorption of solar heat . Melting ice exposes the underlying darker ocean, which then absorbs rather than reflecting sunlight as the bright ice does. The added absorption of solar energy has been equivalent to the addition of 100 billion tons of CO2 to the air. The large warming has exposed a whole new oceanic region in the Arctic.
The California example: California has kept up with the average warming of the planet by about 0.9 ºC, with regions such as the Central Valley warming in excess of 2 ºC. This warming melts the snowpack, and the dark surface underneath absorbs more heat and therefore increases moisture loss by 7–15 percent per degree of warming. This amplified drying becomes chronic, since the warming gets worse each year due to increase in emissions of warming pollutants. The chronic drying is drastically magnified into a mega- drought when rainfall decreases sporadically due to variability in the weather, similar to what has happened over the past four years. The resulting extreme drying of the soil and vegetation contributes to fires. The forest fires, in turn, emit more CO2 as well as black carbon and methane, the two largest contributors to warming next to CO2. This phenomenon is not confined to California. Similar problems are occurring throughout western North America. The melting of northern latitude permafrost and resultant increases in methane emissions are another potential feedback element in warming driven by similar patterns.
With every degree of warming, air holds about 7 percent more moisture. This means that warming is amplified by a factor of two, since water vapor itself is a dominant greenhouse gas [10, 32]. This is one of the most vicious cycles that amplifies greenhouse warming. Increases in water vapor also contribute to extreme storms and increased rainfall, which have become more common, leading to devastating floods around the world.
Climate change directly affects human health through heat waves and increasing frequency and severity of weather extremes such as storms, floods and droughts. Secondary effects include wildfires, worsened air quality, drinking water scarcity and contamination, crop and fishery failures, and expansion of transmissible diseases. Floods, droughts and resource shortages trigger population displacement, mental health effects and potentially violent conflict, both within countries and across borders. Such events will affect poorer nations much more severely, at east initially, but wealthy countries will not be spared significant harm, such as we have already seen from several major hurricanes, floods, droughts and fires in the United States. Within wealthy nations, poor communities will tend to suffer disproportionately from the health effects of climate change.
While the focus of climate change discussions is on CO2 from fossil fuel combustion particulate pollution — nitrogen oxides, toxic pollutants and ozone created from power plants, vehicles and other fossil fuel combustion — also have devastating impacts on human lives and well-being , including:
Direct and Indirect Health Effects of Coal, Petroleum and Gas are also immense and include: Mortality and morbidity; Cardiovascular disease; Acute respiratory infection; Stroke; Mental health; Vector-borne diseases; Water- and food-borne diseases; Heat stroke and other extreme weather related effects; Lung cancer, drowning, under-nutrition; Harmful algal blooms; Mass migration; Decreases in labor productivity . The estimated cost of the health effects is in the range of $70 to $840 per ton of CO2.
One billion of us consume about 50 percent of the fossil fuel energy consumed on Earth and emit about 60 percent of the greenhouse gases; In contrast, the poorest 3 billion, who still rely on pre-industrial era technologies for cooking and heating, contribute only 5 percent to CO2 pollution . Thus, the climate problem is due to unsustainable consumption by just 15 percent of the world’s population. Fixing the problem thus has to simultaneously lower the carbon footprint of the wealthiest 1 billion, while allowing for growth of energy consumption and expansion of carbon sinks, such as forests, needed to empower the poorest 3 billion. It is in this context that it is critical to bend the curve through transforming to carbon neutrality in developed nations while sharing technology that enables developing nations to leapfrog over use of fossil fuels to produce the energy they need . Indeed, for the poorest 3 billion, doing so is literally a matter of life and death.
For example: The poorest 3 billion live mainly in rural areas relying on mixed market and subsistence farming on few acres. A four- year mega-drought of the type that California is experiencing now would change their forms of livelihood and expand the likelihood of both temporary and permanent migration. Small island nations in the tropical Pacific already are facing mass migration caused by increased sea level. If sea level rise reaches 1 meter or more, as is plausible with business as usual, low- lying coastal nations with populations of more than 100 million people — such as Bangladesh — will move to India and other neighboring nations. While likely slower than sudden catastrophic events, the size and scope of such climate migration could make today’s Syrian migration crisis look mild by comparison.
If the carbon footprint of the entire 7 billion became comparable to that of the top 1 billion, global CO2 emissions would increase from the current 38 billion to 150 billion tons every year and we would add a trillion tons every seven years, in turn adding 0.75 ºC warming every seven years. Such impacts mean that children alive today, their children, and their grandchildren, along with all generations to come, will suffer from our unsustainable burning of fossil fuels. What is our responsibility to them?
The 10 solutions in this paper were distilled from the critical analyses provided in the nine companion chapters in this special volume.
The authors have no competing interests to declare.
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